#include <stdio.h>

int main( int argc, char **argv ) {
    unsigned __int64 u64 = 0xffffffff7fffffff;
    double d = *(double*)&u64;

    printf( "%f\n", d );
}

C:\test>cl qnan.c
Microsoft (R) C/C++ Optimizing Compiler Version 15.00.21022.08 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

qnan.c
Microsoft (R) Incremental Linker Version 9.00.21022.08
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:qnan.exe
qnan.obj

C:\test>qnan
-1.#QNAN0